In a conventional image reading apparatus which irradiates an image surface of a document with light emitted by an illumination light source, forms an optical image of the document on a photoelectric conversion element via a mirror and lens, and converts it into an electrical signal, a cold cathode tube, such as a xenon tube, LED or the like is used as the illumination light source.
However, such illumination light source requires a given period of time from when it begins to emit light in response to an ON signal until the amount of light stabilizes. For this reason, in order to perform satisfactory image reading, the control must wait until the amount of light stabilizes, thus often influencing the read time. FIG. 8 shows a change in read level immediately after a lamp is turned on until the amount of light stabilizes. That is, FIG. 8 shows a change in amount of light immediately after the lamp is turned on, the Y-axis plots the amount of light, and the X-axis plots time. As can be seen from FIG. 8, a given time period is required until the amount of light stabilizes, since overshoot or the like occurs immediately after a document illumination lamp is turned on.